Seiches are phenomena of standing oscillation that occur in large lakes, estuaries, and small seas. This condition causes the water within the basin to oscillate much like water sloshing in a bowl.
These oscillations are of relatively long period, extending from minutes in harbors and bays to over 10 hours in the Great Lakes. Any external perturbation to the lake or embayment can force an oscillation. They result primarily from changes in atmospheric pressure and the resultant wind conditions and occur over the entire basin. In harbors, the forcing can be the result of short waves and wave groups at the harbor entrance.